A screw machine is a machine tool in which a tool is moved to engage a rotating workpiece.
A Davenport® multiple-spindle automatic screw machine (available from Davenport Machine, Inc., 167 Ames Street, Rochester, N.Y. 14611) typically has five workpiece-holding spindles that are rotatably indexable from station to station. At each station, a tool is adapted to be selectively moved to engage a rotating workpiece held in the proximate spindle. The tools are carried by tool arms that are pivotally mounted on the revolving head cap of the screw machine. These tool arms are selectively moved relative to the head cap at each station to cause the particular tool held therein to perform a specific machining operation on the relatively-rotating workpiece. Due to their popularity, versatility and adaptability, these machines are in continued use today, and are often upgraded to restore or improve performance.
Heretofore, a shaft (sometimes referred to as a “tool arm stud”) has been positioned between a portion of the tool arm and a portion of the revolving head cap. The shaft was typically formed of a low-carbon low-alloy steel. A bronze bushing was loosely positioned between such relatively-movable portions of the tool arm and the head cap. The diametrical clearance between the bushing and the shaft or head cap was on the order of about 0.001 inches, when newly installed. As the machine was used, normal wear would cause this diametrical clearance to increase. This resulted in increased tool chatter, decreased tool life, looser part tolerances, less-aggressive speeds and feeds of the tool relative to the workpiece, and service intervals of about one week in the most demanding of machining applications.
Accordingly, it would be desirable to reduce the deleterious effects of such increases in diametrical clearance, while keeping costs to a practical minimum. At the same time, it would also be desirable to extend the service life of existing Davenport® screw machines.